Like iSCSI, it exposes a disk image file, or a raw partition if you’d like (by using something like /dev/sda3 or /dev/mapper/foo as the file name). Unlike iSCSI, it’s a fairly basic protocol (the API is literally only 9 commands). iSCSI is essentially just regular SCSI over the network.
NFS and SMB have to deal with file locks, multiple readers and writers concurrently accessing the same file, permissions, etc. That can add a little bit of overhead. With iSCSI and NBD, it assumes only one client is using the file (because it’s impossible for two clients to use the same disk image at the same time - it’ll get corrupted) and it’s just reading and writing raw data.